Whenever I've had issues with the button, whether it was sticking or not functioning, it was a knock-off eGo. Even so, the few times I've had the issue, I've lightly tapped the button on a hard surface like a counter edge or a table edge, LIGHTLY, which loosened it up and fixed the problem.
You can actually tap it quite hard, but make sure not hard enough to damage it. Could just be some little particle in there that needs to be knocked loose.